That’s strange. The weather broadcast should be audible by now. And at this hour, there’s probably no one at the FBO. Great time for a glitch to show up; this is the kind of stunt your instrument instructor would have pulled.

The CFII isn’t along for this ride, but the lessons of those artificially glitchy flights have stayed with you. Now you will have to use a secondary source of altimeter information—and that may mean a higher minimum descent altitude, possibly high enough to make the expected easy approach a closer call.

Checking the approach plate, you see that you will be setting your altimeter to an airport 45 nautical miles away; the note in the upper left corner of the plate says to "increase all MDAs 140 feet."

That seems like a lot. How much would the MDA increase if the remote altimeter setting source (RASS) were closer—say about 30 miles distant?

Surprise: You would increase your minimums 460 feet when shooting a localizer approach to the Mercer County, W.Va., airport, using the altimeter setting at Beckley, 29 miles north. But only increase them 60 feet on this approach to Old Town, Maine, with a RASS from nearby Hancock County-Bar Harbor Airport.

That’s because distance isn’t the only factor the FAA uses to formulate what it calls the RASS adjustment. Another variable in the RASS calculation is the difference in elevation between the airport and the RASS.